The present invention relates to reduction or elimination of uncommanded torque in an electric device that includes, at least in part, a motor and a motor drive, in the event of failure.
Electric devices, that include, for example, a motor, are used for a wide variety of purposes. Many of these devices perform critical functions that affect human safety. For example, electric devices are often used in medical life support equipment or to drive a variety of transporters. Hence, it is often desirable that these devices have a high degree of reliability, and in many cases, a level of redundancy to prevent failure of the device.
Under normal circumstances, motor torque is commonly commanded by a motor drive 51, which regulates the flow of current from a power source 55 into the motor 52, as shown in FIG. 1 (prior art). The motor drive 51 typically includes a controller circuit 53 and a power stage 54, each of which might cause undesired values of current in the motor 52, which in turn causes undesired torque.
In some cases, failures lead to an open-circuited motor (i.e. zero current in the motor), which generally causes a loss of torque from the motor. One way to tolerate this failure mode is to use a motor with redundant windings and redundant drives, all sharing the required load torque, so that in the event of a loss of torque from a single winding and/or drive set, the remaining windings and drives can compensate and meet the necessary torque demands.
However, certain failure modes may cause undesired fault torque that remaining operating windings and drives are not able to overcome, regardless of the amount of redundancy provided. For example, motor terminals may be shorted together by the drive.